The present invention relates to laminators of the type used to laminate a protective film onto a substrate such as an identification card substrate. More specifically, the present invention relates to controlling a heat source used in such laminators.
Laminators are machines that apply a polymer film overlay to a substrate material to improve the durability of the substrate. Laminators are well known for the application of polymeric films to paper. Such laminators are used to apply a thin polymer chip or web to a plastic ID card to improve the durability of the card. Recently, Fargo Electronics, Inc. introduced a printer with an intermediary transfer layer for the production of plastic ID cards. In this printer, a reverse image is printed on a thin polymeric web which is then laminated to the card.
Lamination is generally recognized to be a process requiring a heat source and a machine for applying a pressure to a thin laminate material to laminate a receiving substrate. Commercial laminators used for the lamination of thin films to plastic cards can be standalone machines in which a printed card is fed into the laminator where a overlay film or chip is applied to the card. The laminator can also be integrated into the printer.
Laminators include a heat source such as a hot roller, hot plate, cavity heat source, or thermal print head/platen assembly which is applied to a thin film mechanically positioned to force the film against a receiving card. The heat source typically converts electrical energy to thermal energy with a resistive heating element. This heat conversion process is a slow thermal process. As the card is feed through the laminator the heat source cools quickly and the trailing edge of the card is laminated at a substantially lower temperature. This reduction in temperature can result in poor lamination or adhesion of the laminated layer to the trailing edge of the card.
A conventional approach to controlling the thermal heat source is to use an on-off control strategy, commonly referred to as "bang--bang," where a controller provides a setpoint of the desired laminating temperature to a circuit which controls the heat source. A temperature sensor, typically a thermocouple or RTD, provides temperature feedback to the controller. When the heater temperature arrives at the setpoint, the lamination process starts. When the temperature sensor detects that the laminating heat source has changed due to cooling of the heat source below the card/film lower limit, the controller adjusts the circuit to increase the conversion of electrical power to thermal energy. This type of control is "bang--bang".
Bang--bang control is very simple and cost effective to implement, but often will result in inferior cards as the temperature controller is not able to respond fast enough to the cooling of the heater. The result is a highly variable lamination process parameter along the card.
Further, some laminators must function with a variety of card materials, film materials, and card sizes. The variability introduced by changing thermal masses (card volume) provides a significant challenge to providing high quality laminated cards. Traditional bang-bang control approaches require that a set of lamination parameters be developed for each individual card material/size/laminate and that the lamination temperature and time be adjusted to get the best possible results.
This method is time consuming and sometimes impractical for large enterprises who may run large numbers of different cards and films. Further, the bang-bang control strategy does not provide good process control and laminate adhesion problems result when the heat source is cooled below the required minimum lamination temperature. Attempts to compensate for cooling by starting with a higher lamination temperature result in scorched or melted lamination on the leading edge of the card.
U.S. Pat. No. 5,746,878, issued May 4, 1998 to Kuhns et al., entitled "VARIABLE EFFECTIVE TEMPERATURE LAMINATOR THERMAL BINDER" describes a technique in which the dwell time of a substrate within the laminator is adjusted to achieve a desired level of heating. Further, U.S. Pat. No. 5,203,756, issued Apr. 20, 1993 to Taguchi et al., entitled "LAMINATOR" discusses a laminator in which a phase control type PID controller is used to turn the heating element of the laminator on and off. This design does not provide a regulatory type control signal and the heating of the laminator can only be very controlled.